Gas flow is generally metered by directing the gas through an orifice, measuring its temperature, pressure, specific gravity and the differential pressure across the orifice, and calculating the gas flow using these parameters in the gas flow equation. The American Gas Association's publication "Orifice Metering of Natural Gas" (AGA-3, April 1955, latest reprinting June 1972) sets forth the gas flow equation as follows: EQU Q.sub.h = [F.sub.b F.sub.pb F.sub.tb F.sub.m F.sub.a F.sub.1 ] F.sub.g F.sub.tf F.sub.r Y F.sub.pv .sqroot.h.sub.w P.sub.f
Where:
Q.sub.h = Quantity rate of flow at base conditions: cu.ft./hr PA1 F.sub.b = Base Orifice Factor, a constant for a given orifice size and pipe diameter PA1 F.sub.pb = Pressure Base Factor, a constant PA1 F.sub.tb = Temperature Base Factor, a constant PA1 F.sub.m = Manometer Factor, a constant used only for a mercury manometer PA1 F.sub.a = Orifice Thermal Expansion Factor, a constant over the temperature range of normal interest PA1 F.sub.1 = gauge Location Factor, a constant used only for a mercury manometer PA1 F.sub.g = Specific Gravity Factor PA1 F.sub.tf = Flowing Temperature Factor PA1 F.sub.r = Reynold's Number Factor PA1 Y = expansion Factor PA1 F.sub.pv = Supercompressibility Factor PA1 H.sub.w = Differential Pressure in inches of water PA1 P.sub.f = Absolute Static Pressure in psia PA1 P = pressure in psig PA1 M.sub.c = MOL percent carbon dioxide PA1 M.sub.n = MOL percent nitrogen
For convenience the product of the six "F" terms in brackets in the equation above is represented below by "F." The specific gravity factor and the flowing temperature factor adjust the equation for gases having a specific gravity other than 1.000 and a temperature other than 60.degree. F. as follows: EQU F.sub.g = .sqroot.1/SG,
where SG is the specific gravity of the flowing gas EQU F.sub.t f = .sqroot.520/(T.sub.f +460),
Where T.sub.f is the temperature in degrees Fahrenheit of the flowing gas
The Reynold's number factor takes into account the variations of the discharge coefficient of an orifice with Reynold's number. It is given by: EQU F.sub.r = 1 + (b/.sqroot.h.sub.w P.sub.f),
Where b is a constant for a given orifice size and pipe diameter.
The expansion factor corrects for a change in specific weight when a gas flows through an orifice. It depends on the location and type of pressure taps as follows: EQU Upstream: Y = 1 - K.sub.1 h.sub.w /P.sub.f EQU Downstream: Y = 1 .+-. K.sub.2 h.sub.w /P.sub.f
where K.sub.1 and K.sub.2 are constants for a given orifice size and pipe diameter.
The supercompressibility factor accounts for the deviation of natural gas from the ideal gas laws. The standard American Gas Association method for computing such factor is set forth in its publication "Extension of Range of Supercompressibility Tables" (NX-19, December 1962). This method sets forth a table of F.sub.pv values for adjusted pressures, P.sub.adj, and temperatures, T.sub.adj, where EQU P.sub.adj = [156.47 .times. P] .div. [160.8-(7.22 .times. SG) +M.sub.c -(0.392.times.M.sub.n)] EQU T.sub.adj = [226.29 .times. (T.sub.f + 460)] .div. [99.15 + (211.9 .times. SG) -M.sub.c -(1.681 .times. M.sub.n)] -460
where
Because gas flow through a pipe line typically varies over a very wide range, it has been found desirable to use several orifices to measure flow, an orifice being located in each of several pipes, or "runs," connected in parallel at the point in the pipeline where the gas flow is measured. Thus increases in gas flow through the pipeline are accommodated by opening valves in additional "runs" to permit the gas to flow through. By opening and closing valves in the different runs in step with variations in the gas flow, gas flow through each orifice remains within the range of most accurate measurement. Of course, a separate calculation of the gas flow equation must be made for each different run that is used.
Ordinarily gas flow is calculated every two seconds and the results are accumulated. In practice in the prior art, calculation of the entire gas flow equation every two seconds for as many runs as are used has proven to be too much for the meters used and various terms of the equation have been approximated. Thus, the Reynold's number factor and the expansion factor usually are assumed to be constants and the supercompressibility factor is often approximated by a linear expression. However, with the increasing concern for energy conservation, it has become very important to measure gas flow with the greatest accuracy possible.
At the same time it is often desirable to provide a meter that may be used in different applications with different orifices. Since the orifice constants are different for each case, some means must be provided for entering these constants in the calculator. In the prior art adjustable switches have been used to set different constants for use in calculating the gas flow equation. See, for example, U.S. Pat. No. 3,752,393 to Moseley. However, when one considers that the base orifice factor F.sub.b and the constants, b, and K, used in calculating the Reynold's number factor and the expansion factor are all dependent on the particular orifice used, it is apparent that a large number of switches are needed to enter these constants using the techniques shown in the prior art. In addition, considerable time is required just to read these constants. When one considers that a like number of switches would have to be provided for each run to set the constants in the fashion used in the prior art, it is apparent that the number of switches and the time required to read them are enormous.